4/1/19 Winter Storm Fundamentals Cool-Season Precipitation: Learning Objectives Fundamentals and Applications • After this class you should – Understand the processes that affect the depth and height of the transition zone, the height of the snow level, and the development of snow, ice pellets, freezing rain, or freezing drizzle – Be able to apply top-down forecast approaches for Myoko Kogen, Japan (Photo: Jim Steenburgh) predicting precipitation type Jim Steenburgh Fulbright Visiting Professor of Natural Sciences University of Innsbruck – Understand and apply key meteorological variables Department of Atmospheric Sciences affecting the snow-to-liquid ratio during snowstorms University of Utah [email protected] Ice Particles • Ice crystal – any one of the number of forms in which ice appears Cool-Season Precipitation Types • Ice pellet – Transparent or translucent ice pellet less than 5 mm in diameter (sleet in the US) • Ice pellet aggregate – Individual ice particles linked or fused together Stewart et al. (2015) Ice Particles Ice Particle Examples Ice Pellets (sleet) Snow Snow Pellet (Graupel) • Refrozen wet snow – partially melted snow that refroze • Sleet – In the US, this term refers to ice pellets • Snow – White or translucent ice crystals, chiefly in complex branch hexagonal form and often http://photo.accuweather.com/photogallery/ Steenburgh (2014) agglomerated into snowflakes details/photo/121228/Sleet+Macro Steenburgh (2014) • Snow pellet – White, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, about 2-5 mm in diameter (a.k.a. graupel) Stewart et al. (2015) 1 4/1/19 Type, Size, and Fall Speed Example: Sea Effect of Japan Courtesy Dr. Masaaki Ishizaka Courtesy Dr. Masaaki Ishizaka Example: Sea Effect of Japan Example: Sea Effect of Japan Courtesy Dr. Masaaki Ishizaka Courtesy Dr. Masaaki Ishizaka Mixed-Phase Particles Liquid Particles • Wet snow – Snow that contains a great deal of liquid water • Drizzle – Very small, numerous, and uniformly distributed water drops with diameters < 0.5 mm • Semimelted snow pellet – Snow pellet that has undergone some • Freezing drizzle – Drizzle that falls in liquid form but freezes on impact to form a melting coating of glaze • Freezing rain – Rain that falls in liquid for, but freezes on impact to form a coating • Liquid core pellet – Partially refrozen particle with ice shell and of glaze liquid water core • Rain – Precipitation in the form of liquid water with diameters > 0.5 mm or, if • Almost melted particle – Precipitation composed mainly of liquid widely scattered, drops may be smaller water, but with some ice and the shape not discernible • Supercooled rain – Liquid precipitation at temperatures below freezing • Typically fall speeds increase as particles melt • Typical fall speeds for drizzle ~2m/s; freezing rain, rain, or supercooled rain ~7–9 m/s Stewart et al. (2015) Stewart et al. (2015) 2 4/1/19 The Rain-Snow Transition Zone • A region where falling snow is warming, melting, and becoming rain Z * = Snow ⦿ = “Slush” = Rain • May be identified horizontally or quasi ● horizontally where it intersects the surface, or vertically in soundings or Rain-Snow Transition Zone * * * * mountainous regions Freezing level * * * * Transition * * * * Zone * ⦿ * ⦿ • Becomes a more complex Snow level ⦿ ● ⦿ ● precipitation type transition zone if there are warm layers > 0˚C, which 0ºC can lead to freezing rain or ice pellets T The Rain-Snow Transition Zone The Rain-Snow Transition Zone • Freezing level • Snow level – Glossary of Meteorology: – Glossary of Meteorology: No Z * = Snow Z * = Snow Lowest altitude at which the ⦿ = “Slush” definition provided ⦿ = “Slush” air temperature is 0˚C ● = Rain ● = Rain – Common: Altitude above which – Practical consideration: Critical * * * * snow is accumulating on the * * * * to also know the highest Freezing level * * * * ground Freezing level * * * * Transition * * * * Transition * * * * altitude at which the air Zone * ⦿ * ⦿ Zone * ⦿ * ⦿ Snow level ⦿ ● ⦿ ● Snow level ⦿ ● ⦿ ● temperature is 0˚C – White et al. (2011): Altitude where falling snow melts to rain – Common in aviation to report 0ºC T 0ºC T multiple freezing levels to the highest altitude at which the – Minder et al. (2011): Level at air temperature is 0˚C which the precipitation mass is 50% ice The Rain-Snow Transition Zone Profiling Radar Examples • Snow level – Rule of thumb: ~300 meters below Z * = Snow freezing level ⦿ = “Slush” ● = Rain – Reality: Varies depending on thermodynamic and humidity profile, as well as precipitation * * * * Freezing level * * * * intensity Transition * * * * Zone * ⦿ * ⦿ Snow level ⦿ ● ⦿ ● – As a first guess, some forecasters use the highest level at which the 0ºC wet bulb temperature is 0.5˚C T NOAA/ESRL – Typically lower over mountains than in free air upstream White et al. (2011) 3 4/1/19 Diabatic Effects Z Z Rain/Snow Transition Regions Dr. Ronald Stewart University of Manitoba 2008 AMS/COMET/MSC Mountain Weather Workshop Whistler, British Columbia, Canada 0ºC T 0ºC T Melting creates an isothermal layer Evaporation results in temperatures converging to wet bulb and lower freezing Increased precipitation rates deepen melting and snow levels than indicated by solely layer and lower snow level by temperature Question Isothermal layers at 0ºC are normally __________ over topography than over nearby flat regions a) the same depth b) shallower c) deeper Question Question An increase in precipitation rate could _______ the height of the As one ascends the Seegrubenbahn through the transition region, base of the transition region which sequence best describes the change in precipitation particles with increasing altitude a) raise b) maintain a) snow, wet snow, slush, rain c) lower b) rain, slush, wet snow, snow c) slush, rain, snow, wet snow d) rain, wet snow, slush, snow 4 4/1/19 Pathways to FZDZ, FZRA, & IP • FZDZ: Warm-phase processes (no significant ice nucleation) – Shallow cloud decks forming over arctic airmasses (eastern 2/3 of US) – Cloud top temperatures > -10ºC Pathways to Freezing Drizzle – Poor ice nucleation – Common in central US (FZDZ), Freezing Rain (FZRA) and • FZRA: Deep moist layer and midlevel warm (>0ºC) layer – Cold-air damming with overrunning in eastern US Ice Pellets (IP) – Warm-frontal overrunning in central US – Cold pools, cold-air damming, and terrain-forced flows a factor in western US • IP: See FZRA, but add deeper/colder surface-based layer enabling refreeze before rain reaches ground Rauber et al. (2000) FRZA Climatology: Europe FZRA Climatology • Discussion: Where and • Discussion: In the why are freezing rain continental US, why events most common in are Freezing rain Europe events most common: • In plot – In the Columbia Basin – Orange triangles = number of observed FZRA reports – – Circles = stations > 2000 m In the north-central (omitted) or < 10 FZRA and northeast states? observations – Contours – distance from – East oF the southern coast Appalachian Mountains? Kämäräinen et al. (2017) http://mcc.sws.uiuc.edu/living_wx/icestorms/index.html, https://www.reddit.com/r/MapPorn/ Transition Zone with Melting Layer Example Temperature Profiles Aloft Z Z Z Cloud top 0ºC T 0ºC T 0ºC T Freezing Drizzle Freezing Rain Sleet Poor ice nucleation Snow melts in >0˚C layer; Snow melts in >0˚C layer; + warm-phase processes Rain doesn’t refreeze until Rain refreezes before contacting ground or other objects reaching ground NOAA/NWS 5 4/1/19 You Decide: FZDZ or FZRA? You Decide: FZDZ or FZRA? 95.4% Freezing Drizzle 83.8% Freezing Drizzle 4.6% Freezing Rain 16.2% Freezing Rain Warm rain processes dominate Warm rain processes dominate Rauber et al. (2000) Rauber et al. (2000) You Decide: FZDZ or FZRA? You Decide: FZDZ or FZRA? 72.3% Freezing Drizzle 62.4% Freezing Drizzle 27.7% Freezing Rain 37.6% Freezing Rain Warm rain processes dominate Warm rain processes likely important Rauber et al. (2000) Rauber et al. (2000) You Decide: FZDZ or FZRA? You Decide: FZDZ or FZRA? 53.1% Freezing Drizzle 21.5% Freezing Drizzle 46.8% Freezing Rain 78.5% Freezing Rain Warm rain processes likely important in some cases, Melting of snow from aloft likely dominates ice particles falling into cloud from aloft possible Rauber et al. (2000) Rauber et al. (2000) 6 4/1/19 Top Down Forecasting Top Down Forecasting • A way to assess what hydrometeors will form and • Diagnose potential for melting and freezing their evolution as they fall to the surface during fallout • Frequently based on model or observed – Melting in warm layer? soundings – Freezing in near-surface sub-freezing layer? – Consider diabatic effects (melting, evaporation) • Start at cloud top – Mixed phase or warm phase processes? • CTT ~ -4˚C: Little or no ice • What precipitation will occur at the surface • CTT ~ -10˚C: Commonly used cutoff temp for warm phase – Snow, FZRA, FZDZ, IP? – Beware of seeder-feeder from higher clouds NWSFO, Louisville, KY NWSFO, Louisville, KY You Decide: SN, FZDZ, FZRA, or IP? You Decide: SN, FZDZ, FZRA, or IP? NWSFO, Louisville, KY NWSFO, Louisville, KY You Decide: SN, FZDZ, FZRA, or IP? You Decide: SN, FZDZ, FZRA, or IP? What if elevated convection develops? NWSFO, Louisville, KY NWSFO, Louisville, KY 7 4/1/19 Questions Compared to the free atmospheric upstream, the snow level over the mountains is usually Snow Levels in Mountainous a) the same height b) lower Terrain c) higher Why? The Mountainside Snow Level The Mountainside Snow Level • Compared to free air • Compared to free air upstream, usually near upstream,